Induction heating of metals by utilizing a high-frequency current has been widely used for heat treatment, such as hardening and the like. Induction heating is employed for controlling the quality of the metals when producing thin metal plates and non-ferrous metal plates, such as steel plates and aluminum plates, as well as for increasing productivity by increasing the rate of heating and for freely adjusting the amount of production. Induction heating has been used as a substitute for conventional indirect heating, which uses gas or electricity.
Induction heating of metal plates can be roughly divided into two systems. One is an induction heating system, which is a so-called LF (longitudinal flux heating) system in which a high-frequency current is flown into an induction coil surrounding the metal plate, and the generated magnetic flux passes through the metal plate in the lengthwise direction thereof to induce a current in the sectional surface of the metal plate to heat the metal plate. Another one is a TF (transverse flux heating) system in which a metal plate is arranged between magnetic materials called inductors on which the primary coils are wound, and the magnetic flux generated by flowing a current into the primary coils is permitted to pass through the inductors so that the magnetic flux flowing between the inductors traverse the metal plate so as to induce a current on the plane of the metal plate to thereby heat the metal plate by induction.
Induction heating by the LF system has a good uniformity in the temperature distribution. However, the induced current circulates in the cross section of the plate and when the plate has a small thickness, the current is not induced unless the frequency of the power source is increased due to current penetration depth. In addition, nonmagnetic materials and magnetic materials having a small thickness cannot be heated if the temperature exceeds their Curie points since the penetration depth of current increases.
The induction heating by the TF system, on the other hand, has a feature in that the metal plate can be heated irregardless if it is a magnetic plate or nonmagnetic plate since the magnetic flux passes through the plane of the metal plate. In addition, use of inductors having low reluctance makes it possible to decrease the leakage of magnetic flux and to concentrate the magnetic flux between the inductors facing the front surface and the back surface of the metal plate enabling the heating to be more efficient to be enhanced.
However, there remains a problem in that the temperature distribution tends to be less uniform. In addition, if the metal plate is not at the center between the opposing inductors, the magnetic plate is attracted by either inductor, and the temperature tends to further deviate.
Further, induction heating by the TF system results in defects such as it is difficult to vary the width of the metal plate and it becomes difficult to handle the metal plate in case it meanders on a continuously processing line.
In order to solve these problems, JP-A-2002-43042 discloses coils of a single turn that are arranged in a deviated manner on the front surface and back surface of a band in a direction of travel.
Further, JP-A-2002-151245 proposes an induction coil of a rhombic shape in which an induction heating coil facing the material to be heated has a curved long axis.
In JP-A-2005-209608, the present inventors have proposed an induction coil which surrounds a metal plate and is shifted in a direction of travel.